Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, flash drives, digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
A flash memory device is a type of memory in which the cells are typically grouped into blocks that can be erased and reprogrammed in blocks instead of one byte at a time. Changes in threshold voltage of the memory cells, through erasing or programming of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data value of each cell. The data in a cell of this type is determined by the presence or absence of the charge in the charge storage structure.
A typical flash memory cell comprises a semiconductor on which a dielectric material is formed. The charge storage structure is formed in or adjacent to the dielectric material and a control gate is formed on the dielectric material to control operation of the memory cell. Typical prior art programming uses either Fowler-Nordheim tunneling or hot electron injection to move a charge from a channel in the semiconductor onto the charge storage structure. This type of programming can result in a number of problems.
For example, in order to move the electrons through the dielectric material, a relatively large programming voltage is used (e.g., 20V). Thus, charge pump circuitry is typically used in order to generate the relatively large programming voltages. This circuitry takes up valuable real estate on the integrated circuit. Also, the mechanism by which the electrons tunnel through the tunnel dielectric material damages the dielectric. This limits the number of program/erase cycles that a flash memory device can endure before the dielectric wears out and loses its insulating properties. The memory device then loses its retention capability.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a relatively lower voltage memory that can endure a greater number of program/erase cycles.